This invention relates, in general, to photolithography, and more particularly to photolithography using a contrast enhancement material (CEM) on top of photoresist.
As semiconductor device dimensions approach the submicron level, one of the limiting factors for further reduction in size is the ability to photolithographically reproduce submicron dimensions onto a semiconductor substrate. The minimum dimension which can be reproduced by an alignment tool is determined by the tool's abilities to optically reproduce or image a pattern onto the substrate and to align the imaged pattern to a previous pattern on the semiconductor substrate. The former ability allows the system to produce a small image with acceptable line width control, and the latter ability allows minimum clearance between patterns of a multilayer device. As semiconductor devices become more complex the number of layers used increases and the alignment ability, or alignment tolerance, becomes more important.
A pattern is reproduced on a semiconductor substrate by aligning a mask pattern to a previously formed alignment target or key, and then projecting the mask pattern onto a photoresist covered substrate, thereby exposing the photoresist. The mask pattern comprises an alignment pattern, which is used for alignment, and an active device pattern, which is used to form active devices. Photoresist is sensitive to a narrow bandwidth of light called the actinic wavelengths. The photoresist is also sensitive to the intensity and time of exposure to the actinic wavelength light. Photoresist requires exposure to a threshold energy light for imaging to occur, thus a sub-threshold energy will not expose the photoresist. It is undesirable to expose the photoresist during alignment. One method of aligning the mask pattern to the previously formed target pattern comprises using an alignment wavelength other than the actinic wavelengths for the alignment, so that the photoresist is not exposed during the aligning process. In this manner, the projected alignment pattern can be moved many times with respect to the target until proper alignment is achieved. Once the patterns are aligned, the mask pattern is projected onto the substrate using actinic wavelength light, thereby exposing the photoresist. This method of alignment, however, requires two sets of optics since the focal distances are different for the alignment wavelength and the actinic wavelength. Two sets of optics increase the complexity of the photo alignment tool and decrease alignment accuracy of the tool. Another alignment method which has been developed uses a narrow bandwidth of light of the actinic wavelengths, for both alignment and exposure of the projected pattern. In this method the alignment takes place at a lower energy so that the photoresist layer is not exposed during alignment. Once alignment is complete, the mask pattern is transferred by increasing the light energy above the photoresist's exposure energy threshold. Such systems are called actinic alignment systems. These systems offer the advantage of using a single optic system within the tool and allow for improved alignment tolerances as well as reduced cost and complexity of the tool.
Recently, contrast enhancement materials (CEM) have been used for reproducing images having dimensions around and below 0.5 micron to improve line width control. Contrast enhancement materials comprise a dye which is initially opaque to the actinic wavelengths and bleachable by exposure to light of the actinic wavelength. Above a threshold energy CEM is essentially transparent. CEM may also be responsive to wavelengths other than the actinic wavelength. Contrast enhancement material is applied as a thin layer covering a photoresist layer and, once exposed, forms a portable conformal mask which is in direct contact with the photoresist layer, thus greatly improving the ability to define submicron dimensions. While contrast enhancement materials work well with alignment tools which use non-actinic wavelengths for alignment, when actinic alignment is used the existing pattern on the semiconductor wafer can not be easily seen through the opaque CEM and thus alignment is difficult or impossible. Until now, it has been difficult or impossible to use contrast enhancement material with actinic alignment tools.
One solution to this problem has been to remove the contrast enhancement material over the alignment target by laser ablation so that the alignment target could be seen at actinic wavelengths. Since alignment targets are usually distanced from active device patterns, laser ablation could be used to remove the CEM and photoresist over the alignment target without damaging or exposing active device areas. Laser ablation removes both contrast enhancement material and the underlying photoresist thus exposing the surface of the semiconductor substrate. Laser ablation can result in particulates, generated by the vaporization of the CEM and photoresist, which deposit on active device areas of the substrate. It has also been found that by removing the photoresist layer over the alignment targets that light reflected from the substrate, which is used to detect proper alignment, comprises a large amount of standing wave noise and scattered light which reduce alignment accuracy.
Accordingly, it is an object of the present invention to provide a method for photolithographic alignment which realizes both the improved accuracy of an actinic alignment tool and the small dimensions of contrast enhancement material processes.
It is another object of the present invention to provide a method for actinic alignment using contrast enhancement material with improved alignment tolerance.
A further object of the present invention is to provide a method for actinic alignment using contrast enhancement material with a shorter process time.
It is still a further object of the present invention to provide a method for actinic alignment using contrast enhancement material which is defect free.